The present invention relates in general to the field of heat pipes and, in particular to a new and improved method and apparatus for filling, sealing, and field servicing heat pipes.
Larger size water/carbon steel heat pipes generally incorporate fill tubes through which the working fluid is passed into the heat pipes, and through which the heat pipes are placed under vacuum. In some cases, a union is used to allow for venting of non-condensible gases from the heat pipes after some period of service.
The use of a standard heat pipe fill tube is shown schematically in FIG. 1. During manufacture of the heat pipe 10, one end of a fill tube 12 is welded to a hemispherical or elliptical end cap 14 by means of a weld 16. The heat pipe 10 is then filled with a working fluid schematically indicated at 18, degassed and evacuated to a "hard" vacuum. The heat pipe 10 is then sealed via a tube crimp 20 and a weld 22 at the other end of the fill tube 12. FIG. 2 shows the use of a union or coupling generally designated 30, in combination with two fill tubes, a primary fill tube 32 and a secondary fill tube 34. During manufacture of the heat pipe 10, one end of the primary fill tube 32 is welded to the end cap 14 and the other end is welded to externally threaded portion 36 of union 30. The secondary fill tube 34 is then connected to the primary fill tube 32 via internally threaded portion 38 of union 30. A typical conical bushing 40 fits on a portion 42 of the secondary fill tube 34 within the union or coupling 30 to make the seal. The heat pipe 10 is again filled with the working fluid 18, degassed and evacuated to a "hard" vacuum. The heat pipe 10 is then sealed via a tube crimp 44 and sealing weld 46 on the secondary fill tube 34.
The designs shown in FIGS. 1 and 2 have several drawbacks because each provides for several possible failure points; i.e., at welds 16, 22, 46 and at tube crimp 20, 44. In FIG. 1, the crimp 20 reduces the strength of the fill tube 12, the heat pipe 10 cannot be vented of non-condensible gas after a period of service, and the sampling of non-condensible gas and the making of pressure measurements from a heat pipe 10 requires penetration of the fill tube 12 wall, rendering it unusable as a pressure boundary. In FIG. 2, five possible failure points exist; i.e., at welds 16, 46, at tube crimp 44, and at the seal within union 30. Additionally, venting of the heat pipe 10 in the field is cumbersome, and sampling of non-condensible gas and pressure measurements from the heat pipe 10 again requires penetration of the primary 32 or secondary 34 fill tube wall thus rendering either of them unusable as a pressure boundary. In particular, once any fill tube 12 or 32 is penetrated, the entire heat pipe 10 must be replaced because restoration of the heat pipe 10 in the field is not currently possible.
The manufacturing process for heat pipe 10 also affects the type of fill tube apparatus that can be placed on the end of the heat pipe 10 itself. FIG. 2 represents one known heat pipe 10 construction. It is important to note that some steps in the general manufacturing procedure for this construction involve, inter alia, spinning the heat pipe 10 while the fill tube apparatus is attached. The manufacturing procedure for the heat pipes thus impacts the type of fill tube assembly that can be used because any type of fitting or closure device on the end of the primary fill tube 32 must be axially symmetrical with respect to a longitudinal axis of the heat pipe 10 so that no excessive moment arms occur during spinning of the heat pipe 10. This requirement precludes use of a typical T-type valve having a valve stem and handle which would protrude at an angle from the longitudinal centerline of the heat pipe 10. In addition, such valves could loosen due to vibration during service.
In addition, the heat pipes themselves are charged with a working fluid which, under ambient conditions, is at a vacuum with respect to atmospheric pressure. Thus, any attempt to vent non-condensible gases by merely "cracking" open a union 30 such as shown in FIG. 2, at ambient conditions, would not exhaust gas from the heat pipe, but would rather intake ambient air. For most practical applications, it is thus required to increase the temperature of the heat pipes to a point above 240.degree. F. so that the pressure within the heat pipe is above atmospheric pressure. "Cracking" of the union 30 would thus permit venting of the higher pressure non-condensible gases from the heat pipe to the atmosphere. However, while heating an individual heat pipe might be a relatively straightforward task, these heat pipes are typically part of a larger air heater system wherein such individualized heating is not possible. The entire air heater itself must be elevated in temperature by the use of space heaters and obtaining access for locating same is often extremely difficult. Further, the heat pipes themselves might contain a flammable gas, such as hydrogen, and manually venting same could present a hazard.
Grover (U.S. Pat. No. 4,020,898) and Hoke, Jr. (U.S. Pat. No. 4,799,537) disclose heat pipe apparatus having conventional crimped, soldered or welded end fittings.
Murphy et al. (U.S. Pat. Nos. 4,881,580 and 4,776,389) disclose methods and apparatus for evacuating and filling heat pipes in similar closed vessels. As disclosed in the '580 Murphy et al patent, the heat pipe 16 is processed on a table 14 being held at one end by guides 18, 20 and a clamp 24 having trust bar 26 and thrust finger 28. A block 22 is provided with a process tube 54 at a hex shank 68 which moves the piston 60 having O-ring 64, 66. Integral with the heat pipe 16 is a threaded valve 40 having an axial bore 44 and cross bores 46. The process tube 54 is to provide a vacuum and for filing of the working fluid into the heat pipe 16. Turning the hex shank 68 unscrews the valve 40 from the heat pipe 16 and allows an open passageway to process tube 54. O-rings on the piston 60 seal the apparatus from the atmosphere.
Mahdjuri-Sabet (U.S. Pat. No. 5,241,950) discloses, in essence, safety means for a heat pipe so that damage to the heat pipe due to excessive condenser temperatures is avoided. Referring to FIGS. 1 and 2 thereof, the heat pipe 1 is provided with a transparent jacket that holds the working fluid and the evaporator, and is interconnected by a conduit 4 to the condenser 2. Located within an expanded portion of the condenser 2 is an annular plug 13 encircling an overflow tube 10 which creates a fluid reservoir 12 therebetween. Helical springs 14 and 15 maintain axial forces on the annular plug, but allow it to move when rising working fluid fills the condenser 2 during heat absorption.
Stockman (U.S. Pat. No. 4,341,000) discloses a method of charging a heat pipe whereby a predetermined amount of fluid may be charged into the heat pipe includes a method of changing fluids as necessary. The heat pipe 12 as provided at its upper end a T-fitting 24 through which is provided an inlet working fluid and which also provides for air exhaust. A coupling 26 is used to removably connect the T-fitting 24 to the heat pipe 12. Provided in the internal portion of the heat pipe 12 is a vertical stand pipe 38 whose height is predetermined so that a suction pump 32 connected at a lower end thereof will only be able to remove that portion of liquid above the end termination of the stand pipe 38. The height of the stand pipe 38 can be varied as necessary to provide a predetermined level of liquid in the heat pipe 12.
Hartle et al. (U.S. Pat. No. 5,226,580) is of interest as disclosing an automated heat pipe processing system, wherein a heat pipe casing and an end cap is formed into a heat pipe, then cleaned by means of glow-discharge plasma, filled with a working fluid, and fixing the end cap on the heat pipe by inertia welding.
Franco et al. (U.S. Pat. No. 4,586,561) discloses a low temperature heat pipe employing a hydrogen getter. The term "low temperature" as used in Franco et al. means a temperature below 0.degree. C. (32.degree. F.)at which the heat pipe is operational. The patent discusses one of the largest uses of heat pipes at present being the permafrost stabilization of the trans-Alaskan pipeline. The heat pipes under these conditions are contained in vertical support members that are designed to operate in colder months when the permafrost temperature at moderate depths (20 ft) is above the air temperature. Heat pipes using ammonia as the heat transport medium have been installed using two heat pipes for each vertical support member. During the winter months when the air temperature is below the ground temperature, the heat pipe functions to remove heat from the permafrost thus maintaining its integrity during the subsequent summer months when thawing can potentially occur. A problem with the operation of the heat pipes is the presence of small amounts of non-condensible hydrogen gas which can collect, for example, by a corrosion reaction between water, which may be an impurity in the ammonia and the carbon steel of the pipe. The hydrogen gas accumulates primarily in the condenser section and inhibits the ammonia vapor from condensing at the top of the condensation section. This results in "condenser blockage" and leads to reduced heat removal capability. Thus, the patent is directed to a means or method of removal of such contaminant hydrogen to allow the heat pipe to continue to operate and continue to prevent the permafrost from degrading. Accordingly, the patent discloses a hydrogen getter material, preferably being a zirconium intermetallic alloy, which is effective even in the presence of air and/or water. More specifically, Franco et al. discloses a hydrogen getter assembly for removing contaminant hydrogen gas from an ammonia heat pipe which assembly can be mounted on the pipe on top or on the side, or located inside the pipe on the condensation wall or section. As shown in FIG. 2 of Franco et al., a heat pipe 17 inserted into the permafrost ground 11 has a hydrogen gas getter assembly 23 mounted vertically on top being inserted through cover plate 31. The getter assembly 23 has located therein a getter material 24 contained in getter canister 25 and held in place by retaining element 26 which is sufficiently porous to allow gaseous NH.sub.3 and H.sub.2 through to contact the getter material. FIG. 3 of Franco et al. shows another embodiment of the heat pipe having a getter assembly 23 mounted on the side of the heat pipe 17 rather than on the top. This embodiment is said to provide easier installation of the getter assembly to the heat pipe since it avoids a double-seal penetration process as generally practiced for the assembly of the heat pipe illustrated in FIG. 2 thereof The getter assembly can be attached to the assembled and charged heat pipe (which charging is generally performed under vacuum to avoid the entry of moisture and/or air) by conventional hot tapping methods or non-welding penetration methods. FIG. 4 shows a preferred embodiment of the hydrogen getter assembly 23, wherein the canister housing 25 is inserted in the heat pipe wall 17. Hydrogen and ammonia enter into the interior of the canister 25 by means of the communication inlet 27 and the resulting initial pressure is sufficient to break the rupture disk 29. The getter material is retained in position by retaining element 26 which is porous and permeable to hydrogen and ammonia but is inert and has sufficient strength to provide a barrier to the movement of the getter material into the heat pipe itself In addition, it is stated that there may also be present a valve (not shown) positioned between the canister 25 and exterior condensation wall and operating with the communication inlet 27. The valve 28 is said to be designed to prevent external leakage of ammonia at low temperatures and use of the valve is optional in preparing the heat pipe by non-welding penetration but is preferred when utilizing, for example, hot tapping methods. In addition to the valve in the communication inlet 27, it is stated that there can optimally be joints formed by fittings, such as quick-connects, which allow for closing and detaching the canister after use and protecting the canister contents from air, and the heat tube atmosphere from escaping, during the detachment step.
While Franco et al. discloses that hydrogen getter assemblies can be removably coupled to heat pipes via quick-connects, he neither teaches nor suggests use of such an assembly during the filing, sealing or field servicing of such heat pipes. As indicated earlier, various servicing operations may have to be performed on heat pipes once they have been installed in the field. These include the tasks of: measuring the internal heat pipe pressure to determine how much non-condensible gas is present; obtaining samples of such non-condensible gases for analysis; venting non-condensible gases from the heat pipe; obtaining a sample of the working fluid from the heat pipe for analysis; and performing internal visual inspections of the heat pipe.
Franco et al. is also not particularly concerned with the manufacturing process for heat pipes. Many heat pipes are designed to have spirally wound aluminum fins present on both the evaporator and condenser sections. During the finning process, the heat pipe is spun at a high speed of rotation. Imbalances cannot be tolerated during such finning processes. Further, the addition of fins to heat pipes, particularly carbon steel fins, add significant weight to the heat pipe and therefore it is not practical from a manufacturing standpoint to fill and seal the heat pipe after it has been finned. Additionally, since heat pipes require a specified internal surface cleanliness, finning the heat pipes prior to the welding of the end cap and the like increases the chance of not meeting this requirement due to flash rust concerns. Further, if it is determined that the heat pipes do require cleaning, it would certainly be easier to do this without the fins being present. Finally, the fins- on some portions of heat pipes, particularly the condenser side of the heat pipes, may use aluminum fins which are a much softer material then carbon steel. Such fins would most certainly be damaged beyond repair if manufacture of the heat pipe were completed after the fins were attached to the tube itself.
It is thus clear that an improved method and apparatus for filling, sealing and field servicing individual heat pipes is desirable.